How do you know there's a buy button, then? If the app maker doesn't state in the description or permissions, he's doing everyone a disservice. Like I said - I'd refuse to let the kid have any apps at all - which would bar even the pay-for once apps.
Even if you are monitoring the kid, a phone screen is small. You could simply miss the whole thing. On a larger tablet, like a TV, you might have a better chance.
Younger than 4, you can have the child on your lap and watch. Older kids - that is not so easy. Older kids might have their own phones - just to get in touch with working parents. Or the parents use the location services provided by a carrier.
This happened with a Smurfs game. Parents were really upset. I certainly wouldn't sit there and play the stupid thing.
Now with the Smurfs - I would have suspected. After all, they were commercialized and trademarked. If the game is allied to any trademarked product, I would also suspect. Like Disney. The dev would have to include the copyright symbol for the allied product.
If the info was there, on the description, yes, I would blame myself.
If the game is from an independent, I would have no way of knowing unless it was in the description or permissions.
If I want to purchase anything from Amazon, I have to sign it. Why can't Google have you sign in for a purchase? Having to do that at Amazon hasn't stopped anyone.
On the same vein - how would you stop an older kid from downloading adult material or really violent games? These are rated in the physical world and a storekeeper can deny the purchase.